Health Hazards of Chemicals and Chemical Exposure
The health effects of hazardous chemicals are often less clear than the physical hazards. Data on the health effects of chemical exposure, especially from chronic exposure, are often incomplete. When discussing the health effects of chemicals, two terms are often used interchangeably - toxicity and hazard. However, the actual meanings of these words are quite different. Toxicity is the ability of a chemical substance to cause harm. Hazard is the likelihood that a material will cause harm under the conditions of use. Thus, with proper handling, even highly toxic chemicals can be used safely. Conversely, less toxic chemicals can be extremely hazardous if handled improperly.
The actual health risk of a chemical depends on the toxicity and the actual exposure. No matter how toxic the material may be, there is little risk involved unless it enters the body. An assessment of the chemical's toxicity and the possible routes of entry will help determine what protective measures should be taken.
The toxic effects of a chemical may be local or systemic. Local injuries involve the area of the body in contact with the chemical. For example, if you spill an acid on your arm, the effect will be on your arm. Systemic injuries involve tissues or organs other than the contact site where toxins have been transported through the bloodstream. For example, methanol that has been swallowed may cause blindness.
Certain chemicals may affect a target organ. For example, lead primarily affects the brain, kidneys, and red blood cells and some solvents may harm the liver and kidneys.
It is important to distinguish between acute and chronic exposure and toxicity. Acute toxicity results from a single, short exposure. Effects usually appear quickly and are usually reversible. Chronic toxicity results from repeated exposure over a long period of time. Effects are usually delayed and gradual, and may be irreversible. For example, the acute effect of drinking alcohol is becoming drunk, while the chronic effect from drinking alcohol over a long period of time is cirrhosis of the liver.
Some people may be more or less sensitive to specific chemicals, depending on several factors including eating habits, physical condition, obesity, medical conditions, drinking and smoking, and pregnancy.
Over a period of time, regular exposure to some substances can lead to the development of an allergic rash, breathing difficulty, or other reactions. This phenomenon is referred to as sensitization. Over time, these effects may occur with exposure to smaller and smaller amounts of the chemical, but will disappear soon after the exposure stops. For reasons not fully understood, not everyone exposed to a sensitizer will experience this reaction. Examples of sensitizers include epoxy resins, nickel salts, isocyanates, and formaldehyde.
Many chemicals have been evaluated for their ability to cause cancer. The latency period for most cancers range from twenty to forty years. The risk of developing cancer from exposure to a chemical increases with the length of exposure and with the exposure concentration.
It is important to understand the distinction between human carcinogens and suspected human carcinogens. The term human carcinogen is used when there is clear evidence of the ability to cause cancer in humans. Suspected human carcinogen refers to chemicals that have been shown to cause cancer in two or more animal species and are, therefore, suspect in humans.
Anyone who works with, or plans to work with carcinogens or suspected carcinogens must follow strict guidelines to minimize exposure. For a particular substance, the Toxicological Information section of the Safety Data Sheet will state whether or not the substance is considered a carcinogen by the Occupational Safety and Health Administration (OSHA), the National Toxicology Program (NTP), or the International Agency for Research on Cancer (IARC).
Reproductive toxins are chemicals which affect the reproductive system, including mutagens (those which cause chromosomal damage), teratogens and embryotoxins. Embryotoxins may be lethal to the fertilized egg, embryo or fetus, may be teratogenic (able to cause fetal malformations), may retard growth or may cause post-natal functional deficits. Other reproductive toxins may cause sterility or may affect sperm motility.
Some chemicals may cross the placenta, affecting the fetus. A developing fetus may be more sensitive to some chemicals than its pregnant mother, particularly during the first twelve weeks of pregnancy, when the mother may not know she is pregnant. Proper handling of chemicals and use of protective equipment is especially important to reduce fetal exposure to chemicals.
Known human teratogens include organic mercury compounds, lead compounds, ionizing radiation, some drugs, alcohol, and cigarette smoking. Some substances which may cause adverse reproductive effects in males include 1,2-dibromo-3-chloropropane, cadmium, mercury, boron, lead, some pesticides, and some drugs. More than 800 chemicals have been shown to be teratogenic in animal models - many of these are suspected human teratogens.
Individuals who work with teratogens and who are contemplating pregnancy or are pregnant should review the toxicity of the chemicals in their workplace and may consult with EHS to determine whether any of the materials pose additional risk during pregnancy. The California Proposition 65 list includes a wide range of naturally occurring and synthetic chemicals that are known to cause cancer or birth defects or other reproductive harm.